1. Field of the Invention
This invention relates to a battery array that connects many individual battery cells in series or parallel, and in particular relates primarily to a battery array used as a power source apparatus to power an electric motor that drives an automobile.
2. Description of the Related Art
An electric vehicle such as an electric automobile or hybrid car, which is powered by both an internal combustion engine and an electric motor, uses a battery array of many connected individual battery cells as a power source to supply electric power to the motor for driving.
A battery array used in this type of application should establish high output voltage to supply power to a high output motor. Consequently, many individual battery cells are connected in series and housed in a holder case. For example, battery arrays installed in hybrid cars currently on the market connect several hundred individual battery cells in series to produce output voltages of several hundred volts. In these battery arrays, five or six individual battery cells are connected in series to form a battery module, and many battery modules are housed in a holder case.
A battery array installed in an electric vehicle such as a hybrid car discharges at high currents to speed up the motor when the car accelerates rapidly. In addition, the battery array is charged with high currents via regenerative braking when decelerating or traveling downhill. Consequently, battery temperature can become considerably high. Since use extends to the hot environment of summer months as well, battery temperature can increase even further. Therefore, it is important for a battery array housing many batteries in a holder case to provide efficient and uniform cooling of each battery inside. Various problems arise if temperature differentials develop between the batteries being cooled. For example, a battery that gets hot can degrade and its actual charge capacity at full charge will decrease. If a battery with reduced charge capacity is connected in series and is charged and discharged with the same current as other batteries, it can easily be over-charged or over-discharged. This is because the capacity to which the degraded battery can be fully charged and the capacity that can be completely discharged become smaller. Battery characteristics degrade dramatically with over-charging and over-discharging. Consequently, a battery with reduced actual charge capacity degrades in an accelerated fashion. In particular, if the battery's temperature becomes high, degradation is further increased. As a result, uniform cooling that generates no temperature differentials over any of the batteries is important for a battery array housing many batteries in a holder case.
Various system configurations have been developed to realize this objective (refer to Japanese Patent Laid-Open Publication 2001-313090; Japanese Patent Laid-Open Publication 2002-50412; and Japanese Patent Laid-Open Publication HEI 11-329518 (1999)).
The battery arrays disclosed in JP2001-313090A and JP2002-50412A were previously developed by the present applicant. In those battery arrays, battery modules, which connected a plurality of individual battery cells in a linear fashion, were housed in a parallel orientation within a holder case. Inside the holder case, battery modules were cooled by the forced flow of cooling air across the battery modules. Battery modules were disposed in two levels in the direction of the cooling air flow. Further, a plurality of holder cases was arranged inside an outer case. In such a battery array, output voltage can be adjusted by the number of holder cases housed in the outer case. In addition, each holder case had cooling gaps established between the holder case and the battery modules housed inside. Battery modules were cooled by forcing cooling air flow through the cooling gaps. To cool each battery module uniformly, structural components were disposed between the battery modules stacked in the direction of air flow to control the flow of cooling air.
A battery array with this (prior art) configuration can uniformly cool battery modules stacked in two levels and housed in a holder case. However, if battery modules are stacked inside a holder case in three levels or more to reduce the overall battery array footprint, this type of battery array becomes unable to uniformly cool each battery module.
JP H11-329518 (1999) describes a battery array that houses three levels or more of battery modules inside a holder case. This battery array houses multiple levels of battery modules inside a holder case with battery modules oriented in a parallel fashion and separated in the direction of cooling air flow. This battery array cools battery modules by forcing cooling air flow between battery modules. In this cooling configuration, cooling efficiency is lower for downstream battery modules than for upstream battery modules, and downstream battery modules develop higher temperatures. To resolve this drawback, a turbulence-inducing element such as a dummy battery unit is provided at the most upstream position of the holder case. By disturbing the flow of cooling air introduced into the holder case, battery modules in upstream locations are efficiently cooled. As a configuration to increases cooling efficiency for batteries located downstream, supplemental cooling air intakes are provided in the holder case to capture cooling air at intermediate locations along cooling air pathways.
In this battery array, the cooling efficiency of upstream and downstream battery modules can be improved respectively by turbulent flow and by injecting cooling air at intermediate locations. However, all battery modules cannot be cooled to a uniform temperature with this configuration.
The present invention was developed to further resolve these drawbacks. Thus, it is a primary object of the present invention to provide a battery array that can reduce temperature differentials between a plurality of battery modules housed in a holder case, and can cool battery modules uniformly.